Method and device for operating a steam turbine comprising several no-load or light-load phases

ABSTRACT

The invention relates to a method and a device for operating a steam turbine ( 10 ) comprising several no-load or light-load phases ( 11, 12 ). All phases ( 11, 12 ) are supplied with steam in order to ensure good preheating. According to the invention, the supply of a phase ( 11 ) is selected in such a way that said phase ( 11 ) produces the least possible output, in particular no output. The enthalpy differential (Δh) between the entrance ( 25 ) to and exit ( 26 ) from the phase ( 11 ) is thus preferably reduced to zero.

[0001] The present invention relates to a method for operating a steamturbine, which has a plurality of stages, during idling or low-loadoperation with steam being admitted to all the stages. It also relatesto a device for distributing steam to individual stages of a steamturbine during idling or low-load operation, in particular for carryingout the method mentioned.

[0002] Steam turbines and their design problems are, in particular,presented in Prof. Dr.-Ing. H.-J. Thomas, “Thermische Kraftanlagen”[Thermal Power Installations], 2^(nd) Edition, 1985, Springer-Verlag.Details for calculating the enthalpy and further thermodynamicparameters can, for example, be extracted from “Technische Formeln furdie Praxis” [Technical Equations for Practical Use], 24^(th) Edition,1984, VEB Fachbuchverlag, Leipzig.

[0003] Further reduction in the starting times of steam turbines iscontinuously required. Shorter starting times can only be achieved ifall stages have, as far as possible, the largest possible mass flowadmitted to them at the same time. It is only by this admission that thepreheating of the steam turbine necessary for the shortest possiblestarting time can be achieved. The power generated by the turbine due tothe mass flow being admitted must not, however, exceed the idling load.If the idling load is exceeded, uncontrolled increases in the rotationalspeed of the steam turbine can occur. The total mass flow which can besupplied overall is, therefore, limited.

[0004] High windage powers occur at the exhaust-steam end of thehigh-pressure stage (HP stage) during idling or low-load operation.These high windage powers lead to high temperatures at the exhaust steamend. A large part of the mass flow must therefore be supplied to thehigh-pressure stage in order to prevent unallowably high temperatures.The low-pressure stage (LP stage), however, also demands a comparativelyhigh mass flow, in particular where large low-pressure stage crosssections and new materials, for example titanium for the blading of thelow-pressure stage, are employed. The medium-pressure stage (MP stage)also requires a part of the mass flow.

[0005] If the necessary, high mass flow is admitted to both thehigh-pressure stage and the low-pressure stage, the overall powergenerated is distinctly located above the idling power. Attempts havetherefore been made to adjust the distribution of the mass flows, bymeans of preliminary calculation, in such a way that idling operationbecomes possible. In this case, the mass flows through the high-pressurestage and the medium-pressure/low-pressure stage were distributed insuch a way that the power was not located above the idling powerrequired. It was only overheating of the high-pressure stage which wasavoided by monitoring the temperature occurring at the exhaust-steamend. Only a small mass flow was left for themedium-pressure/low-pressure stage. If the mass flow for themedium-pressure/low-pressure stage was not sufficient or if thetemperature at the exhaust-steam end of the high-pressure stage exceededa specified value, rapid partial shut-down of the high-pressure stagewas initiated. In consequence, the high-pressure stage, at least, wasonly inadequately preheated. Because of this inadequate preheating, alonger starting time was necessarily involved.

[0006] The object of the present invention is, therefore, to makeavailable a method and a device which permit good preheating of all thestages of a steam turbine without exceeding the load at idling or thatin low-load operation.

[0007] In a method of the type mentioned at the beginning, this objectis achieved—according to the invention—by the admission to a stage beingselected in such a way that this stage delivers as little power aspossible.

[0008] Steam can be admitted to all the stages of the steam turbine bymeans of the method according to the invention. The admission takesplace in such a way that a stage delivers as little power as possible.This stage therefore generates only a small amount of power so that acomparatively large mass flow can be admitted to the remaining stages.All the stages are therefore reliably preheated so that short startingtimes can be realized.

[0009] Advantageous embodiments and developments of the invention aregiven by the subclaims.

[0010] The enthalpy of the steam at inlet into this stage and theenthalpy of the steam at outlet from this stage are advantageouslydetermined and the enthalpy difference between inlet and outlet isadvantageously minimized. The power delivered by a stage is directlyproportional to the enthalpy difference. By minimizing the enthalpydifference, therefore, the power delivered can be minimized at the samemass flow or even an increased mass flow.

[0011] According to an advantageous development, the temperature of thesteam at inlet into this stage and the temperature of the steam atoutlet from this stage are measured and the enthalpy difference betweeninlet and outlet is determined, in particular calculated, from thesetemperatures. The temperature of the steam is easy to measure so thatthe measurement complexity is reduced.

[0012] In order to increase the accuracy, the pressure drop between theinlet into this stage and the outlet from this stage is, advantageously,additionally measured and is taken into account in the calculation ofthe enthalpy difference between inlet and outlet. The enthalpy of thesteam flowing through the stage depends on both the pressure and thetemperature. The enthalpy difference can be more accurately determined,in particular calculated, by taking account of pressure and temperaturethan it can by taking account of the temperature alone.

[0013] In another advantageous development, the enthalpy of the steam atinlet into this stage and the enthalpy of the steam at outlet from thisstage are measured. A suitable method for measuring the enthalpy ofsteam is, for example, described in WO 99/15887 by the presentapplicant. This publication refers to DE-B 10 46 068 for determining theenthalpy of live steam, i.e. of superheated steam. In contrast, WO99/15887 relates to a measurement and calculation method for determiningthe enthalpy of wet steam. In order to extract a sample, a partialvolume flow of the wet steam is brought together with a reference gas soas to form a mixture and so that the liquid constituents of the partialvolume flow evaporate completely. Using measured physical parameters,the enthalpy of the reference gas and the enthalpy of the mixture aredetermined and the enthalpy of the wet steam is calculated from them.The information revealed by WO 99/15887 and DE-B 10 46 068 is to beexpressly encompassed in the content of the present application.

[0014] In an advantageous embodiment, the mass flow supplied to thisstage is modified in order to minimize the enthalpy difference. The massflow supplied generates power due to expansion in the front part of thisstage. At the exhaust-steam end, the mass flow is compressed again andconsumes power by this means. By modifying the mass flow supplied, abalance can be found between the two processes and the enthalpydifference can be minimized by this means.

[0015] The admission to this stage is advantageously regulated in such away that this stage does not deliver any power. For this purpose, it isnecessary to regulate to zero the enthalpy difference between inlet andoutlet. The mass flow through this stage therefore provides no power andis only used for preheating. It is then possible to admit the completemass flow to the further stages of the steam turbine in order toovercome the idling load. The maximum mass flow is therefore admitted toall the stages and they are preheated in an optimum manner. The startingtimes can therefore be substantially reduced.

[0016] In a device, of the type mentioned at the beginning, for theachievement of the object, provision is made according to the inventionfor the device to have a first measuring station for recording theenthalpy of the mass flow supplied to a stage, a second measuringstation for recording the enthalpy of the mass flow emerging from thisstage, a comparison unit for determining the enthalpy difference and aunit for adjusting the mass flow supplied to this stage.

[0017] The device according to the invention permits a determination ofthe enthalpy difference, either by means of a direct measurement of therespectively present enthalpies or by means of a measurement ofparameters relevant to the enthalpy, such as pressure and temperature.The enthalpy difference determined can be regulated by means of the unitfor adjusting the mass flow supplied.

[0018] The invention is described in more detail below using exemplaryembodiments which are represented in a diagrammatic manner in thedrawing. In the drawing, the same designations have been used forsimilar components or components which are functionally identical. Inthe drawing:

[0019]FIG. 1 shows a diagrammatic representation of a steam turbine; and

[0020]FIG. 2 shows an enlarged representation of the high-pressurestage, in a second embodiment.

[0021]FIG. 1 represents a steam turbine 10 with a high-pressure stage 11and a combined medium-pressure/low-pressure stage 12. The stages 11 and12 are connected together by means of a shaft 13, which drives agenerator 14 in order to generate electrical current. The shaft 13 andthe generator 14 can be decoupled from one another by means of anappliance, which is not represented in any more detail. A steamgenerator 15 is used for generating the steam necessary for operationand during idling. A condenser 16 for condensing the emerging steam isprovided downstream of the medium-pressure/low-pressure stage 12. Thecondensate is returned to the steam generator 15 via pumps 17, amedium-pressure/low-pressure preheater 18 and two high-pressurepreheaters 19 and 20. A reheat system 21 and a feed-water preheatingsystem A, B, C, D, n are provided to increase the efficiency duringoperation. The components mentioned, and their functions, are known tothe specialist so that it is possible to dispense with a more detailedexplanation.

[0022] The steam generator 15 makes available a mass flow {dot over(m)}. The mass flow {dot over (m)} is subdivided upstream of thehigh-pressure stage 11. A first mass flow {dot over (m)}₁ is supplied tothe high-pressure stage 11, while the remaining mass flow {dot over(m)}₂ is supplied directly to the reheat system 21, bypassing thehigh-pressure stage 11. A mass flow {dot over (m)}₃ is admitted to themedium-pressure/low-pressure stage 12. The remaining mass flow {dot over(m)}₄ is guided directly to the condenser 16, bypassing themedium-pressure/low-pressure stage 12. Valves 22, 23 and 24 are used foradjusting the mass flows {dot over (m)}₁ and {dot over (m)}₃. The massflows {dot over (m)}₂ and {dot over (m)}₄ follow automatically from theadjustment of the mass flows {dot over (m)}₁ and {dot over (m)}₃.

[0023] A first measuring station 25 is provided upstream of thehigh-pressure stage 11 and a second measuring station 26 is provideddownstream. In the case of the usual assumption of an isentropicexpansion, the power P generated by the high-pressure stage 11 is givenby:

p={dot over (m)} ₁ (h ₂ −h ₁)={dot over (m)}₁Δh

[0024] where {dot over (m)}₁ is the mass flow

[0025] h₁ is the enthalpy at measuring station 25

[0026] h₂ is the enthalpy at measuring station 26

[0027] Δh is the enthalpy difference between measuring stations 26 and25

[0028] Because the mass flow {dot over (m)}₁ through the high-pressurestage 11 is constant in steady-state operation, the power P is directlyproportional to the enthalpy difference Δh. With the exception ofmechanical losses, this power is also delivered. In order to minimizethe power P delivered, it is therefore necessary to minimize theenthalpy difference Δh, if possible bringing it to Δh=0.

[0029] In the exemplary embodiment represented in FIG. 1, thetemperature T₁ of the mass flow {dot over (m)}₁ entering as steam intothe high-pressure stage 11 is measured at the measuring station 25. Atemperature measurement takes place downstream at the measuring station26, a temperature T₂, the exhaust steam temperature from thehigh-pressure stage 11, being determined at this measuring station 26.The pressure difference Δp between the measuring stations 25 and 26 isadvantageously determined simultaneously by means of suitable pressuremeasuring appliances (not specified in any more detail). The measuredtemperatures T₁ and T₂, together with the measured pressure differenceΔp, are supplied to a control unit 27, which calculates the enthalpydifference Δh between the measuring stations 25 and 26. The valve 22 isactivated as a function of the result of the calculation, so that themass flow {dot over (m)}₁ is regulated as a function of the calculatedenthalpy difference Δh. This balance for the high-pressure stage 11 isessentially achieved by the exhaust steam temperature T₂ being held (bythe control circuit 27, which provides a valve trimming dependent on theenthalpy) to a value which corresponds to the throttled live steamtemperature. A mass flow {dot over (m)}₁ with a correspondinglythrottled temperature T₁ is therefore made available and supplied to thehigh-pressure stage 11 by throttling the steam mass flow {dot over (m)}by means of the valve 22. The throttling action (throttling effect) ofthe valve 22 is, in this arrangement, employed in a targeted manner inorder to adjust the desired temperatures T₁ and T₂

[0030] In this procedure, a calculation of the enthalpy difference Δh isunderstood to mean not only the actual calculation of this enthalpydifference Δh but also any other appropriate process, by means of whichthe enthalpy difference Δh can be minimized. As an example, a comparisoncan be made with a table which is programmed within the control unit 27.

[0031] The enthalpy difference Δh determines the power P generated bythe high-pressure stage. By means of the valve 23, therefore, thecontrol unit 27 controls the mass flow {dot over (m)}₃ through themedium-pressure/low-pressure stage 12, corresponding to a specifiedidling load and the power generated by the high-pressure stage 11.Further measuring stations for recording temperature and/or pressure canbe provided downstream of the reheat system or at other suitablepositions in order to increase the accuracy.

[0032]FIG. 2 shows an enlarged representation of the high-pressure stage11, together with the associated control of the mass flow {dot over(m)}₁. In the exemplary embodiment of FIG. 2, the enthalpies h₁ and h₂are measured directly at the measuring stations 25 and 26 and theenthalpy difference Δh is subsequently formed in the control unit 27.The valves 22 and 23 are activated by the control unit 27 on the basisof the enthalpy difference Δh. By this means, the power P delivered bythe high-pressure stage 11 is minimized and the mass flow {dot over(m)}₃ through the medium-pressure/low-pressure stage 12 issimultaneously maximized.

[0033] The admission, provided according to the invention, to thehigh-pressure stage takes place in such a way that as little power P aspossible, and advantageously no power at all, is delivered. The methodpermits an admission to all the stages 11 and 12 of the respectivelymaximum possible mass flow {dot over (m)}₁, {dot over (m)}₃. By thismeans, good preheating of all the stages 11 and 12 and, therefore, shortstarting times are achieved. Exceeding the idling load and anunallowable increase in the rotational speed of the steam turbine 10 arereliably avoided.

1. A method for operating a steam turbine (10), which has a plurality ofstages (11, 12), during idling or low-load operation with steam beingadmitted to all the stages (11, 12), characterized in that the admissionto a stage (11) is selected in such a way that this stage (11) deliversas little power as possible.
 2. The method as claimed in claim 1,characterized in that the enthalpy (h₁) of the steam at inlet (25) intothis stage (11) and the enthalpy (h₂) of the steam at outlet (26) fromthis stage (11) are determined and the enthalpy difference (Δh) betweeninlet (25) and outlet (26) is minimized.
 3. The method as claimed inclaim 2, characterized in that the temperature (T₁) of the steam atinlet (25) into this stage (11) and the temperature (T₂) of the steam atoutlet (26) from this stage (11) are measured and the enthalpydifference (Δh) between inlet (25) and outlet (26) is calculated fromthese temperatures.
 4. The method as claimed in claim 3, characterizedin that the pressure drop (Δp) between the inlet (25) into this stage(11) and the outlet (26) from this stage (11) is additionally measuredand is taken into account in the calculation of the enthalpy difference(Δh) between inlet (25) and outlet (26).
 5. The method as claimed inclaim 2, characterized in that the enthalpy (h₁) of the steam at inlet(25) into this stage (11) and the enthalpy (h₂) of the steam at outlet(26) from this stage (11) are measured.
 6. The method as claimed in oneof claims 1 to 5, characterized in that the mass flow ({dot over (m)}₁)supplied to this stage (11) is modified in order to minimize theenthalpy difference (Δh).
 7. The method as claimed in one of claims 1 to6, characterized in that the admission to this stage (11) is regulatedin such a way that this stage (11) does not deliver any power.
 8. Adevice for distributing steam to individual stages (11, 12) of a steamturbine (10) during idling or low-load operation, in particular forcarrying out the method as claimed in one of the preceding claims,characterized in that the device has a first measuring station (25) forrecording the enthalpy (h₁) of the mass flow ({dot over (m)}₁) suppliedto a stage (11), a second measuring station (26) for recording theenthalpy (h₂) of the mass flow ({dot over (m)}₁) emerging from thisstage (11), a comparison unit (27) for determining the enthalpydifference (Δh) and a unit (22) for adjusting the mass flow ({dot over(m)}₁) supplied to this stage (11).